Liquid packaging board is a type of paperboard manufactured in the paper industry for packaging liquids such as milk and fruit juices. The product is made using one or more layers of bleached fibers and has requirements of high stiffness and good smoothness. After calendering, in which typically one side is finished to a target smoothness, both sides of the paperboard are coated, e.g., with polyethylene.
Liquid packaging board is also used in the manufacture of aseptic packaging wherein the board is laminated to a foil, e.g., aluminum foil. This laminate is then polyethylene-coated to provide a starting material for the aseptic package. In addition, liquid packaging board is used in other multilayer structures for manufacturing shelf-stable and hot-fill packaging.
The method commonly employed for obtaining good smoothness on this grade of paperboard is to calender the board in multiroll calenders referred to as wet and dry stacks. The process entails overdrying the sheet to obtain a flat moisture profile of 1-2% and then passing it through the wet stack, where water is added to the sheet in one or more calender nips using water boxes. The added moisture and applied pressure in the nips tend to develop good smoothness for the sheet. The moisture pickup is typically greater than 10-12% of the conditioned weight of the paperboard and can sometimes be as much as 15-18% of the conditioned weight of the paperboard. The sheet is then dried in an intercalender region where there can be one or more driers to remove the moisture picked up in the wet stack. The board is then passed through another multiroll stack, with one or more nips where the smoothness is further developed. One of the advantages of waterboxes is that the water applied can have other functional additives such as dyes, lubricants, binders such as starch and film formers such as polyvinyl alcohol.
While the process described is used in several existing manufacturing facilities of liquid packaging board, it has several limitations. First, overdrying of the sheet to reduce the incoming moisture into the wet stack causes the production to be slower if the drying capacity is limited. Even if the drying capacity is not limiting, overdrying involves the cost associated with drying the grade to the targeted moisture levels. Second, the waterboxes present several operational problems, including difficulty during threading and a tendency to cause breaks. Finally, calendering in several nips with a high moisture content in the sheet densifies the web significantly. In other words, the caliper and hence the flexural stiffness are significantly reduced in the wet and dry stacks. The stiffness reduction is compensated by producing the board with more fiber.
In view of the foregoing, alternative methods for improving the smoothness of the board without sacrificing bulk and stiffness are of interest. Smoothness can be developed by allowing the cellulose fibers to replicate a smooth finishing surface. This can be accomplished by heating the fibers to a temperature higher than the glass transition temperature of the fibers and pressing the fibers to a smooth surface. On the other hand, bulk preservation is expected to be better at lower temperatures, where the web is relatively incompressible. The effect of web temperature on bulk preservation of basestock used for liquid packaging board is shown in FIG. 1. The data in FIG. 1 shows that the cooler web (at approximately 80.degree. F.) must be calendered at a much higher line load than the hot web (at approximately 160.degree. F.) to achieve similar caliper reduction.
Temperature gradient calendering is a known process where the surface of the board is heated to a temperature higher than the glass transition temperature of the cellulose in the nip while the temperature of the sheet is substantially cooler. This process enables smoothness development with reduced bulk loss compared to regular machine calendering. In addition, surface moisturization can also be used to lower the glass transition temperature preferentially closer to the surface to develop smoothness without sacrificing bulk. The effect of sheet temperature on bulk preservation during temperature and moisture gradient calendering is shown in FIGS. 2 and 3. The data in FIGS. 2 and 3 shows that for a given Parker smoothness, the caliper that can be attained using a cold web is higher.
Soft calendering, another method of calendering used primarily for coated substrates, also relies on the temperature gradient calendering concept but the web that is being pressed against a hot surface in a nip is supported by a roll that has a resilient cover. The resilient cover gives the paper a longer dwell time in the nip compared to hard steel nips and also allows the smoothness and gloss development to occur at relatively uniform density across the width of the paper. Soft calendering is an expensive option for existing machines and has limitations, such as cover delamination and cracking due to overheating.
A new type of calendering apparatus that extends the soft calendering concept to longer nip widths and reduces the operational problems has been described in recent patent literature. This apparatus is referred to as extended nip calendering and uses an endless band/belt over a backing roll to provide support for the paper web that is pressed against a heated cylinder. Another variation to this concept is to use a shoe instead of a roll as a backing for the paperboard. The backing shoe provides longer nip widths and hence an increased dwell time.